An auxiliary power unit (APU) system is often provided on an aircraft and is operable to provide auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a brushless, synchronous generator is coupled to and driven by the gas turbine engine during operation in a generating mode whereupon the generator develops electrical power.
As is known, an electromagnetic machine may be operated as a motor to convert electrical power into motive power. Thus, in those applications where a source of motive power is required for engine starting, such as in an APU system, it is possible to omit the dedicated starter motor and operate the generator as a motor during the starting sequence to accelerate the engine to self-sustaining speed. This capability is particularly advantageous in aircraft applications where size and weight must be held to a minimum.
The use of a generator in starting and generating modes in an aircraft application has been realized in a variable-speed, constant-frequency (VSCF) power generating system. In such a system a brushless, three-phase synchronous generator operates in the generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
The generator of such a VSCF system is operated as a motor in the starting mode to convert electrical power supplied by an external AC power source into motive power which is provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion field windings via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
In order to properly accelerate the generator and prime mover during operation in the starting mode, it is necessary to properly commutate or switch the currents among the armature windings of the generator. In the past, proper commutation was achieved using an absolute position sensor, such as a resolver, a synchro, an optical encoder or hall effect devices. For example, in Lafuze, U.S. Pat. No. 3,902,073 three Hall sensors are mounted in an air gap of a PMG 120 electrical degrees apart with respect to permanent magnet rotor pole pairs. As the rotor of the PMG rotates, the voltage output of each Hall sensor switches on and off as a function of the rotor position thereby generating three square wave voltages spaced apart by 120 electrical degrees. The outputs from the Hall sensors are representative of the position of the PMG rotor and are used to control switching elements in cyclo-converters to switch current to armature windings of a main generator portion.
Use of an external absolute position sensor entails considerable expense in the position sensor itself and associated electronics, and further results in extra wires and the need for an extra assembly step to install the components. Also, operational parameters often limit the accuracy of the sensor.
In view of the foregoing difficulties, other approaches have been taken in an effort to detect rotor position without the need for absolute position sensors. In the case of a brushless DC motor control, a back EMF approach has been used to detect rotor position. The back EMF of the motor is defined by the following equation: EQU E.sub.emf =K.omega.Sin.alpha.
where K is a constant, .omega. is the angular speed of the motor and .alpha. is the electrical phase angle of the rotor. From the foregoing equation, it can be seen that if back EMF can be detected, rotor electrical phase angle can be determined and thus proper commutation of the armature windings of the motor can be achieved. The back EMF voltage can be detected using either of two methods, referred to as the direct method and the indirect method.
The direct method can be used to directly measure phase back EMF voltage only when the phase winding is not energized by the inverter connected thereto and when the winding is not short circuited either by closed switches in the inverter or by conducting flyback diodes in the inverter. Such conditions can be realized when a 120 degree commutation algorithm is utilized. In this case, a voltage reading is taken after a short delay following switching of the phase winding off to ensure complete current decay by the free-wheeling diodes. This direct technique is described in a paper entitled "Microcomputer Control for Sensorless Brushless Motor" by E. Iizuka et al., IEEE Transactions on Industry Application, Vol. IA-21, No. 4, May/June 1985.
The indirect method is based on estimating the back EMF from the motor terminal voltage and phase currents. This method is suitable for both 120 and 180 degree commutation algorithms. One technique that uses this method is described in a paper entitled "Position--and--Velocity Sensorless Control for Brushless DC Motor Using an Adaptive Sliding Mode Observer" by Furuhashi et al., IEEE Transactions on Industrial Electronics, Vol. 39, No. 2, April 1992.
Because the back EMF voltage of a motor is zero at standstill and the signal to noise ratio is small at lower speeds, the reliable determination of rotor position by detecting back EMF is limited at low rotor speeds.
A method of using a permanent magnet generator as a position sensor for motor/generator start is described in Stacey U.S. Pat. No. 5,140,245. A standard brushless generator is equipped with a PMG which is used as an emergency electric power source and as a source of control power during a normal or generating mode of operation. The PMG develops a multi-phase output which is supplied to a high resolution phase-locked loop having a binary counter which develops an output signal representing shaft position. This method, however, is limited to the situation where the number of PMG rotor poles is equal to or less than the number of poles on the main generator portion rotor so that ambiguous position readings are avoided.